(S)-4-benzyl-3-butyryl-2-oxazolidinone (1.65 g, 6.68 mmol) was dissolved in 22 ml of dehydrated methylene chloride, which was cooled to −74° C. under an atmosphere of argon. After triethylamine (1.11 ml, 8.02 mmol) was added, 1.0 mol/l solution of dibutylboryltrifurate in methylene chloride (7.35 ml, 7.35 mmol) was added dropwise over 15 minutes, which was stirred for 30 minutes. Next, after stirring for 50 minutes under cooling with ice, the mixture was cooled to −75° C. Following this, benzyl 5-formyl-2-methoxybenzoate (1.81 g, 6.68 mmol) dissolved into 56.5 ml of dehydrated methylene chloride was added dropwise over 20 minutes. After stirring for 1.5 hours at −75° C., the mixture was stirred for 2 hours under cooling with ice. A mixed solution comprising 30 ml of methanol, 16.7 ml of phosphate buffer and 7.3 ml of 30% aqueous hydrogen peroxide was added and the mixture was stirred further for 30 minutes at 0° C. The reaction mixture was poured into water, which was extracted with methylene chloride. The extract was washed with brine, then dried over anhydrous magnesium sulfate and concentrated. The residue was purified by silica gel chromatography (eluate n-hexane:ethyl acetate=3:2 v/v) to obtain 1.36 g (39%) of (4S)-3-[3-(3-benzyloxycarbonyl-4-methoxyphenyl)-2-ethyl-3-hydroxypropionyl]-4-benzyl-2-oxazolidinone.
